In many fields of technology it is necessary to have rather precise information concerning a movement process. In particular in automated, controlled, or regulated systems, only these data permit reliable and efficient operation. A system of this type is, for example, a turbocharger in a motor vehicle. Through knowledge of the precise rotary motion of the compressor wheel not only the rotary speed of the turbocharger can be monitored and regulated according to the predefined theoretical values, but also an optimization of the efficiency of the entire motor can be effected. For as precise a representation of the rotary speed as possible, the individual blades of the compressor wheel are detected. In so doing, eddy currents frequently come into use in practice.
Blades of a compressor wheel of turbochargers normally consist predominantly of aluminum or, in particular in the case of commercial vehicles, titanium. These blades are very thin and usually have, e.g., in the case of titanium blades, a thickness of approximately only 0.5 to 0.7 mm. Depending on the manufacturing tolerances, in the detection of the blades with an eddy current sensor a detection spacing of approximately 2 mm must be adhered to. This can be achieved in the case of standard eddy current sensors only with correspondingly large coil diameters.
In the measuring device known from practice the coil is disposed in such a manner that the coil axis is oriented so as to be perpendicular to the direction of movement of the blade wheel. In this way, relatively high signal edges can be achieved. However, in the case of very narrow blades, an undesired effect occurs which makes the signal evaluation significantly more difficult. If a blade is moved past a sensor, there is an impedance change whose signal curve has the form of a double peak.
In connection with this, the signal trough is greatest when the blade is positioned so as to be centered above the sensor. This trough is strongly dependent on the blade thickness—the thinner the stronger—and on the electrical conductivity of the blade material—the lower the conductivity the deeper the trough.
To carry out an unambiguous detection process, a signal evaluation is thus carried out. Since, however, at the highest rotary speeds signal frequencies up to approximately 1 MHz have to be processed, the costs for the evaluation electronics increase significantly. This is too expensive, in particular for use in standard production.
The present invention is thus based on the objective of developing and extending a device of the type stated in the introduction in such a manner that detecting flat test objects can be done in a simple and economical manner. A corresponding process will be specified.